Zener diodes are bipolar devices that have a p-n junction with high carrier densities. They utilize breakdown (avalanche breakdown or Zener breakdown) that takes place under reverse bias. When the breakdown voltage is exceeded, the diodes keep the voltage between terminals constant irrespective of the current (hereinafter, the Zener voltage).
Zener diodes having silicon (Si) or the like are known. However, silicon power semiconductor devices have reached the limits of silicon properties, and enhancing the performances thereof is difficult.
Silicon carbide (SiC) is a promising semiconductor material for power devices due to its favorable electrical and physical properties. For example, a breakdown strength and a thermal conductivity of SiC are almost ten times and three times higher than those of Si, respectively.
SiC p-n diodes are well known as SiC bipolar devices (Patent Document 1). For example, compared to a Si p-n diode with a breakdown voltage of 10 kV, a SiC p-n diode with a breakdown voltage of 10 kV has an approximately ⅓ forward voltage and an approximately 1/20 or less reverse recovery time, and can reduce electric loss to approximately ⅕ or less, thereby greatly contributing to energy saving.
SiC bipolar devices other than SiC p-n diodes, for example SiC n-p-n transistors, SiC SIAFET and SiC SIJFET are also reported to reduce electric loss.    Patent Document 1: JP-A-2002-185015
To obtain SiC Zener diodes having a high current capacity, it is necessary that the breakdown current flows uniformly through the entire p-n junction. In SiC Zener diodes having a mesa structure, however, it has been found that electric fields are concentrated in a region where the p-n junction exposes to aside face of the mesa wall (hereinafter, the region will be referred to as a p-n junction end on the mesa wall) as shown in an image and a sketch thereof in FIG. 2 and FIG. 3, respectively. As a result, a current is conducted locally and a high current capacity cannot be obtained. This problem will be described in detail below.
Depletion layers formed in biased p-n diodes behave differently in Zener diodes and p-n diodes other than Zener diodes. In p-n diodes other than Zener diodes, a depletion layer formed at a p-n junction grows in the thickness direction of a first conductivity type layer with increasing voltage and finally reaches the bottom of the first conductivity type layer. This behavior is because the doping density of the first conductivity type layer except the substrate is low in the range of 1014 to 1016 cm−3 and the depletion layer extends easily. The diode sustains the applied voltage through the thickness direction of the first conductivity type layer (the thickness of the first conductivity type layer except the substrate ranges from several tens of μm to 300 μm), and therefore the diodes can withstand a high voltage. However, when a depletion layer formed at the p-n junction extends beyond a mesa corner, electric fields are concentrated at the mesa corner. This causes breakdown at the mesa corner, and consequently the withstand voltage is limited. Herein, the “mesa corner” indicates a region of a conductive layer and a junction termination extension in a p-n diode in which a flat region around the mesa structure (hereinafter, the mesa bottom) and a side face of the mesa structure cross each other. As an example, FIG. 4 shows an image obtained by analyzing an electric field distribution in a p-n diode other than Zener diodes by electric field simulation. FIG. 5 which is a sketch of FIG. 4 shows regions where the electric fields are stronger in the order of E11>E10>E9>E8>E7>E6. FIG. 5 indicates that the electric field is high at the p-n junction, and in particular at the mesa corner.
In Zener diodes, the doping density in the first conductivity type layer except the substrate is usually high in the range of 5×1016 to 2×1019 cm−3, and a depletion layer formed at the p-n junction do not substantially grow. Further, the thickness of the depletion layer is not more than 0.5 μm in diodes having a Zener voltage of 100 V or less, and is not more than 2.5 μm in diodes having a Zener voltage of 400 V or less. It has been then found that because the depletion layer in the Zener diodes is unlikely to reach the mesa corner in contrast to the p-n diodes other than the Zener diodes, the electric field concentration occurs not at the mesa corner but at the p-n junction end on the mesa wall.
The present invention is aimed at solving the problems described above. It is therefore an object of the invention to provide SiC Zener diodes that are free of electric field concentration at the p-n junction end on the mesa wall and thereby have a high current capacity.